1. Field of the Invention
This invention relates to a unique fabric woven with a combination of conductive and non-conductive fibers that functions as a touch sensor. This invention further relates to a method for connecting control electronics to the fabric that are able to measure the location and determine the contact area of multiple, simultaneous touch events on the fabric.
2. Statement of the Problem
Touch input sensors and controllers are becoming common as user input devices to control computers, cell phones, and other electronic devices. When a touch sensor is transparent and placed in front of a display, the sensor-display assembly is a touch-screen. A touch sensor without a display can be opaque and is a touch-pad.
Different technologies exploit various parameters of layered materials to make sensors. Resistive sensors are constructed to hold two linearly resistive layers apart at rest that are allowed to come into contact upon a touch event. The resistor dividers resulting from the touch are read by control electronics and a location is calculated. Capacitive sensors are constructed from conductive layers that are driven by signals to generate an electric field. When the base capacitance is altered by another electric field, such as a user's finger, the differences are measured by control electronics and a location is calculated. Inductive sensors are driven by signals to generate a magnetic field. When the magnetic field is altered by another magnetic field, such as a stylus with a coil, the differences are measured by control electronics and a location is calculated. Surface Acoustic Wave sensors use transducers to initiate mechanical waves in a sensor, and process the reflected wave pattern to look for changes caused by touch points reflecting and/or absorbing the wave energy. IR sensors shine a grid of beams across the sensor surface that is monitored on opposing sides. A touch breaks the beam and the control electronics determine a location.
Of these sensor technologies, resistive has distinct advantages. Resistive sensors are like mechanical switches so respond to touches from all actuators including fingers, even when gloved, and common implements such as pencils and pens. A touch is not ambiguous because the same pressure that informs the electronics of a touch informs the user of a touch. Resistive also has a high signal to noise ratio allowing for the possibility of high resolution.
Major problems of resistive touch sensors are optical clarity when used as a touch-screen in front of a display and robust function in the field for either touch-screen or touch-pad applications. This invention focuses on touch-pad applications for the technology so optical clarity is not important, though those skilled in the art can apply this invention to touch-screens. The problem with the robustness of traditional resistive sensors is due to the need to maintain a thin air gap between the layers of the sensor at rest, but allow the layers to come into contact with one another upon a touch event.
Maintaining a thin air gap is particularly difficult when the sensor is large. For example, when the sensor is used as a large screen for a projected display in an interactive white board application. Maintaining a thin air gap is also a problem when the sensor needs to conform to a non-planer surface such as when draped on a user's leg or applied to a contoured surface in an automobile.
One way to overcome these air gap challenges is to make a sensor by weaving fibers with a specialty cross section made up of conductive and non-conducive regions into a fabric where the non-conductive regions keep the conductive regions of crossing fibers apart when at rest but allow the conductive regions to come into contact under the pressure of a touch. The specialty cross section, however, introduces complexity to the weaving process as the orientation of the fiber in the weave needs to be controlled. Also, non-linearities in the resistance of the conductive region of the fiber hinders the ability of control electronics to accurately determine touch locations.